chemisty Yr 9
chemisty Yr 9
History of the atom
Atoms were originally thought to be hard tiny spheres
When electrons were discovered a "plum pudding" model was suggested
Alpha Partick scattering led to the nuclear model
Later protons were discovered
James Chadwick discovered neutrons
Radius of an atom : 1x10^-10m
Proton mass 1
neutron mass 1
electron mass 0
Mass No (total number of protons+neutrons)=23
Atomic (proton) No (Number of protons)=11
Isotopes are atoms that have the same No of protons (atomic number) but a different number of neutrons (mass number) eg. 37/17 Cl and 35/17 Cl
Electrons in an atom occupy energy levels or shells
Metals form positive ions by losing electrons
Non metals form negative ions by gaining electrons
Ions have full outer shells (noble gas configuration)
Elements are arranged by their proton number.
Elements in the same group have similar properties. They have the same amount of electrons in their outer shell.
Elements used to be arranged by atomic mass. Mendeleev left gaps for undiscovered elements. When protons were discovered his periodic table fitted the pattern.
Noble gases/group 0 are unreactive because they have a full outer shell. The boiling point increases down the group as the atoms become heavier.
Alkali metals/group 1 gets more reactive as you move down because the outer electron is further from positive pull of the nucleus and is easily lost
Group 1 with water:
Li - floats on the surface, fizzes melts.
2Li+ 2 H2O> 2LiOH+H2
Na- Floats on the surface, more rapidly, fizzes, melts 2Na + 2H2O >2NaOH +H2
K- Floats, fizzes, melts.
Group 1 with air
Li burns to form Li2O
Group 1 with Chlorine
All group 1 metals react vigorously with chlorine
Reactions get more vigorous as you go down the group
Group 7: the halogens
All have 7 electrons in their outer shell.
As you go down the melting point increases and reactivity decreases
A more reactive halogen displaces a less reactive halogen from a solution
Are harder, stronger, have higher melting points and densities than group 1 metals. They are also less reactive
Bonding structure and properties of matter
Ionic Bonds- between a metal and non metal , oppositely charged ions.
Metals form positive ions by losing electrons and non metals for negative ions by gaining electrons
The ionic bond is the electrostatic attraction between the positive and negative ions
Ionic compounds are giant ionic lattices held together by strong electrostatic forces between the oppositely charged ions
Ionic compounds have high melting and boiling points as a lot of energy is needed to overcome the strong ionic bonds. They don't conduct electricity when solid but do when molten of dissolved in water and the ions are free to move and carry charge
Covalent bonds- between 2 non metals, sharing of electrons
Simple covalent molecules: have strong covalent bonds between the atoms but weak forces between the molecules (weak intemolecular forces
Examples; NH3, H2,Cl2,O2,N2, HCl, H2O,CH4
Properties of simple molecules, Usually gases or liquids with low melting and boiling points as the forces between the molecules are weak and these need little energy to break. The covalent bonds between the atoms remain unchanged. The bigger the molecule the higher the melting/boiling point as the inter molecular forces increase. Covalent molecules do not conduct electricity as they have no free electrons or ions.
Giant covalent substances:have strong bonds throughout giants lattice structures extending in 3 dimensions.
Examples; Diamond, graphite, silicon dioxide (sand) ,graphene, fullerenes
Diamond (same for sand) each carbon atom in diamond is covalently bonded to 4 others in a giant lattice. This makes diamond very hard with a very high melting point because the strong covalent bonds require a great deal of energy to break. Diamond does not conduct electricity because is has no delocalised electrons and no ions.
Graphite has a layered structure. Within each layer each carbon atom is bonded to 3 other carbon atoms strong covalent forces between the layers. Each carbon atom has 1 spare electron that is free to move and carry charge so graphite can conduct electricity. Graphite has a high melting point because it has strong covalent bonds that need a great deal of energy to break.
Graphite is slippery and can be used as a lubricant in pencil 'leads' as the layers can slide over each other due to the weak forces between the layers.
Graphene is a single layer of graphite that can conduct electricity yet is incredibly thin but strong.
Fullerenes are molecules of carbon atoms that make hollow shapes that can be used to carry drugs inside the body and for electronics. Carbon nanotubes are long thin fullerenes that are useful for strengthening materials.
Polymers are long chains of small molecules (monomers) joined together. The atoms in a polymer chain are covalently bonded. The forces between each polymer chain are weaker.
Metallic bonds - in metals
Metals consist of giant structures of a regular arrangement of ions surrounded by a sea of delocalised electrons. The electrons are free to move and carry charge so metals can conduct electricity and energy. The attraction between the positive metal ions and the delocalised electrons is strong so metals have a high melting and boiling points. The regular arrangement of ions in layers mean that metals ca be bent and shaped as the layer can slide over each other
Nanoscience is the science of very small things. Nanoparticles have a high SA:V ratio so smaller amounts can often be used. They are used in cosmetics and electronics. The advantage is that less can be used but further research needed into long term effects